1. Field of the Invention
This invention generally relates to systems and methods configured for processing a substrate with a heated fluid and, more specifically, to systems and methods incorporating microwave heaters within fluid supply lines of systems used to process substrates.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Many applications utilize heated fluids for processing a substrate. Examples of processes include but are not limited to film deposition techniques, such as electroless plating and electroplating, and microelectronic device fabrication processes, such as depositing, etching, activating, polishing, cleaning, rinsing, and drying. In some systems, fluids may be heated within a chamber configured for processing the substrate. In other cases, however, it may be advantageous to additionally or alternatively heat fluids prior to being supplied to the chamber. For instance, it may be beneficial to preheat a fluid such that the time used to bring the fluid up to the desired temperature within the chamber may be minimized. Conventional systems which heat fluids prior to being supplied to a chamber typically include electric resistive-element heaters or fluid heat exchangers arranged about a fluid storage tank and/or along a supply line coupled to the chamber. Although such heating systems may be suitable for many applications, they do have drawbacks.
In particular, since electric resistive-element heaters and fluid heat exchangers function by transferring heat through walls of the unit around which they are arranged, the boundary layer of a fluid near the walls of the unit may have a substantially higher temperature than a central stream of the fluid. In addition, electric resistive-element heaters, such as electric jacket heaters or screen-printed heaters, may be prone to producing hot spots along a unit wall due to a lack of uniformity of resistive element placement along the surface of the unit (i.e., the resistive elements may not be distributed evenly across the surface). Fluid temperature variations resulting from such characteristics of electric resistive-element heaters and fluid heat exchangers may be particularly undesirable for applications which require a tight temperature tolerance for processing a substrate uniformly.
In addition, temperature variations may be undesirable for fluids which are unstable or degrade at high temperatures. For instance, many electroless plating solutions are susceptible to decomposing at high temperatures, which alters the properties of the solution and, in effect, changes the rate and uniformity of film deposition or, in some cases, halts film deposition entirely. More specifically, high temperatures within an electroless plating solution may cause components to vaporize, dissociate, or react. In some cases, high temperatures may cause metal ions within an electroless plating solution to plate out (i.e., the metal ions may be transformed into elemental metal or metal compound particles). The transformation of the metal ions may reduce the availability of such ions to be deposited upon a substrate catalytic surface and, in some cases, may further or alternatively cause defects and/or particles to be formed.
Although electric resistive-element heaters and fluid heat exchangers may be tailored to heat a wall of a fluid storage tank or a supply line below the stability-threshold decomposition temperature of a fluid contained therein, such adaptations may undesirably limit the fluid to attain an overall desired processing temperature. In particular, since a central portion of a fluid within a fluid storage tank or a supply line may generally be heated to a lower temperature than boundary layers of the fluid at the walls of an electric resistive-element heater or a fluid heat exchanger, the overall temperature of the fluid may be heated to a lower than a targeted temperature. In some applications, low temperatures may hinder the rate of performance of the processes. For instance, deposition rates of some electroless plating processes may decrease relative to decreases in the temperatures of deposition solutions used for the processes. As such, there is a trade-off with using electric resistive-element heaters and fluid heat exchangers to heat fluids for some processing applications. Although electric resistive-element heaters and fluid heat exchangers may, in some embodiments, be sized to minimize wall temperature for a given temperature increase of a fluid (and, thus, minimize issues of temperature variation and high wall temperatures), large equipment is undesirable for many applications, such as in microelectronic fabrication where it is advantageous to minimize the area occupied by process equipment.
Another detriment of electric resistive-element heaters and fluid heat exchangers is that they have significant thermal mass. More specifically, electric resistive-element heaters and fluid heat exchangers require a significant amount of time to heat up to a desired temperature as well as a significant amount of time to cool down once power is disconnected from the units. Such time constraints may be undesirable for many applications. In particular, the time needed to heat an electric resistive-element heater or a fluid heat exchanger to a desired temperature may undesirably delay production time. Furthermore, in embodiments in which a fluid is routed through a heating system while its temperature is being raised, the fluid either has to be circulated or wasted. Discarding unused fluids increases processing costs and, in some cases, increases environmental hazard exposures. Circulating a fluid at elevated temperatures, however, may be undesirable for some applications.
For instance, many electroless plating solutions are susceptible to degrading faster over a given amount of time when exposed to elevated temperatures. As such, even though a solution may be heated by an electric resistive-element heater or a fluid heat exchanger to a temperature below a stability-threshold decomposition temperature, the solution may degrade faster since it is at an elevated temperature for a longer amount of time during a circulation cycle. Exposure to elevated temperatures may also occur when a heating system is cooling down due to the high thermal retention of electric resistive-element heaters and fluid heat exchangers. Although fluid may be flushed from lines for a cool down mode and, consequently, exposure to elevated temperatures may be limited to fluid stored within a heated storage tank, emergency shut downs or breakdowns of systems can trap fluid in supply lines, exposing the stagnant fluid to elevated temperatures.
As such, it would be advantageous to develop a system and a method for heating a fluid prior to being supplied to a chamber for processing without disrupting the rate of the process and/or advancing the decomposition of the fluid. In addition, it would be beneficial for such a system and method to minimize or eliminate high wall temperatures and temperature variation within the fluid. Such a system and method may be advantageous for any process which utilizes a heated fluid to process a substrate, including but not limited to electroless plating, electroplating, and various microelectronic device fabrication processes.